Gas metal arc (GMA) welding and gas tungsten arc (GTA) welding processes are well known in the art. In both processes, heat from an electrical arc melts a consumable filler wire to form a weld. In the GMA process, a filler wire is fed through a welding gun or torch assembly, and the filler wire itself is electrified to form an arc between an end of the filler wire and a work piece. As the end of the filler wire melts away, the filler wire is advanced to maintain the filler wire end proximate to the work piece. In a specialization of the GMA process known as flux cored arc (FCA) welding, the filler wire has a flux core, the flux containing various alloying and cleaning agents.
In the GTA process, an arc is created between a non-consumable tungsten electrode, held by the welding gun or torch assembly, and the work piece. Heat from the arc melts the work piece, forming a weld pool. A filler wire may be applied to the weld pool to add material to the weld.
In both the GMA and GTA processes, a shielding gas is typically used. Various shielding gasses are used for either process, although different gasses or gas mixtures are generally chosen for each process, and specialized gas mixtures may be employed for certain types of welds or for welding certain metals.
Certain advantages and disadvantages are found with each of the processes. The GTA process typically results in a preferential bead contour and appearance, and in fewer imperfections. The GTA process, however, is generally more time consuming because of a lower rate of filler metal deposition. The GMA process provides a greater deposition rate, and provides a generally acceptable bead profile for fill and cap passes. The FCA process provides the greatest deposition rate, an acceptable bead profile for fill and cover passes, and heat affected zone metallurgical qualities similar to the GMA process. The FCA process, however, requires slag removal between passes and produces the greatest amount of fumes.
Given the differences between the processes, a single welding task may employ more than a single process. For example, in certain welding procedures employed in joining pipes, the GTA welding process may be initially used for one or more passes, and a weld joint finished using one or more GMA passes to fill and cover the weld joint.
Various semi-automated, automated or robotic welding apparatus are known for performing complex, time consuming, or precision welds. One type of welding apparatus is specialized for welding pipes: orbital welding.
Orbital welding apparatus employ a welding head that travels about the circumference of a pipe, often riding along a track placed about the pipe. Known orbital welding apparatus employ either a GTA welding head or a GMA welding head. Thus, for welding tasks wherein both GTA and GMA processes are to be employed, it is necessary to acquire both GTA and GMA welding equipment systems.
To perform initial GTA welding passes followed by GMA welding passes requires, at a minimum, that the welding head be replaced for each process. Further, shielding gas supplies and power supplies must be exchanged to meet the different needs of the GTA and GMA processes. Moreover, issues of compatibility between the weld head and the track may require that a track installed for the GTA process must be removed altogether, and a track compatible with the GMA weld head must be replaced in alignment with the pipe joint. Weld track placement, in an acceptable manner, can be a time consuming and tedious task. It can be recognized that the need to acquire and maintain separate GTA and GMA welding equipment, along with the need to change equipment during the course of a welding task, results in increased cost of completing a welding task that requires both GTA and GMA welding processes.
For the foregoing reasons, there is a need for a welding apparatus that is adaptable to perform both GTA and GMA welding processes.